RE: [chrony-dev] pps source is still marked as synchronised when no absolute clock sources are available

[Bill Unruh <unruh@xxxxxxxxxxxxxx>]

On Thu, 21 Jan 2010, Hattink, Tjalling (FINT) wrote:

> > On Wed, Jan 20, 2010 at 04:36:17PM +0100, Hattink, Tjalling (FINT)
> wrote:
> > > I agree with you that chrony will be able to keep time between the
> pps
> > > pulses. But in my situation I cannot blindly trust the incoming PPS
> > > signal being the correct time. When the GPS card has no lock it keeps
>
> > > generating PPS pulses from its own oscillator, but it will be free
> > > running and drifting fast as no GPS corrections come in. So somehow I
>
> > > have to tell chrony not to trust the PPS source anymore when GPS lock
>
> > > is gone. I used to do this for ntpd by making the SHM clock
> > > unreachable, but it works differenty in chrony.
> >
> > I agree that sometimes it may be dangerous to keep using PPS signal
> without
> > any reference, but sometimes it's useful. When local stratum is
> enabled,
> > you can even use PPS signal without any reference and correct the
> seconds
> > by wristwatch if needed.
> >
> > Ideally, this would be configurable. Perhaps a refclock option to bind
> it
> > with another one. This would also allowed to use PPS signal immediately
> > without waiting for first sync.
>
> This would be a really nice solution. It gives the user full control in
> how the PPS should be treated. I can bind the PPS signal to the SHM
> reference clock, which makes it clear to chrony that the PPS signal
> should only be trusted when the SHM clock is trusted. If no binding is
> made chrony should always trust the PPS signal.

I think this adds more complexity to chrony, when it belongs in the refclock
software. In your case both the pps and the nmea come from the same device,
and both are good or bad. That single device should be delivering a single
time, not two times, to chrony. Ie, the software which reads the gps should be
reading both the pps and the nmea and delivering one time to for example the
shm driver. One should not complicate chrony to make up for deficiencies in
the driver. One job per program.


> > > If you would like to always trust any incoming PPS signals, even if
> no
> > > absolute reference source is available, maybe I can solve it by
> > > implementing a chronyc command to disable specific reference clocks.
> >
> > Suspend/resume refclock commands would be certainly useful also in
> other
> > cases.
>
> I was already wondering why there were no commands in chronyc for
> configuring the reference clocks, as there are available for ntp
> sources.

Because refclocks were added only a few months ago.


> --
> Tjalling Hattink
>
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--
William G. Unruh   |  Canadian Institute for|     Tel: +1(604)822-3273
Physics&Astronomy  |     Advanced Research  |     Fax: +1(604)822-5324
UBC, Vancouver,BC  |   Program in Cosmology |     unruh@xxxxxxxxxxxxxx
Canada V6T 1Z1     |      and Gravity       |  www.theory.physics.ubc.ca/

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